a) Field of the Invention
The invention is directed to a method for energy regulation of pulsed gas discharge-coupled radiation sources, particularly of excimer lasers, F2-lasers and EUV radiation sources based on a gas discharge. It is applied particularly in semiconductor lithography for chip fabrication.
b) Description of the Related Art
In addition to special lamps, narrowband excimer lasers with wavelengths of 248 nm and 193 nm are currently used as radiation sources for producing microchips. Scanners based on F2 lasers (157 nm) are in development.
In all photolithography processes, a mask (containing the structure to be imaged) is imaged on the wafer in the scanner in a reduced manner (the reduction is typically 1:5). EUV lithography (at around 13.5 nm) appears to be the most promising variant for next-generation lithography.
Aside from the characteristics of the optical system (numerical aperture, depth of focus, aberrations or imaging errors of the lenses or mirrors), the image quality of the photolithographic process is essentially determined by how accurately the radiated radiation dose can be maintained. According to V. Banine et al. (Proc. SPIE Vol. 3997 (2000) 126), this dose stability (dose accuracy) is determined by:
a) pulse quantization
b) pulse-to-pulse stability
c) spatial stability of the emitting volume.
Pulse quantization is scanner-specific. The quantity of light pulses that can fall into the moving slit during a scan varies. However, this quantity can usually be ignored.
The quantities b and c are specific to the radiation sources themselves. Quantity c is significant only for EUV sources based on detectable fluctuations of the emitting plasma.
The requirements of the chip manufacturer with respect to dose stability (at the wafer site) place extremely high demands on pulse-to-pulse stability. This is expressed in the standard deviation σ of the actual light pulse energy from the average light pulse energy or from the target pulse energy value (set energy). For narrowband excimer lasers, DUV lithography and VUV lithography require σ-values of less than 1.5% and EUV lithography even requires σ-values of less than 0.4%.
These demands can only be met by means of pulse-to-pulse energy regulation. Pulse-to-pulse energy regulation for pulse train frequencies in the kHz range is only possible by means of a fast high-voltage regulation of the charging voltage U.
In control engineering, PID (proportional-integral-differential) controllers are used very often for controlling processes. PI (proportional-integral) regulation is somewhat simpler and, in many cases, more stable. PI regulation was also described in U.S. Pat. No. 6,005,879 for fast pulse energy control of narrowband excimer lasers. The charging voltage is regulated for the first 10 . . . 40 pulses in an exposure burst in a modified PI regulation which, however, retains the empirical regulation factors.
U.S. Pat. Nos. 5,440,578, 5,450,436 and 5,586,134 also disclose fast pulse control, but are directed to the interplay between the regulation of high-voltage and gas supply for pulse energy stabilization in excimer lasers rather than to the cyclical processing of measurement values.
A fast pulse regulation means controlling the pulse energy of every laser shot by controlled variation of the charging voltage. The algorithm of PI control commonly permits calculation of the charging voltage U for the pulse energy E of the light pulse n according to the following formula:                                                                                           E                  n                                =                                                      E                                          n                      -                      1                                                        +                                      A                    ⁡                                          (                                                                        E                          S                                                -                                                  E                                                      n                            -                            1                                                                                              )                                                        +                                      B                    ⁢                                                                                   ⁢                                          D                                              n                        -                        1                                                                                                        ,                                                                          where              ⁢                                                                                                                                                         D                                      n                    -                    1                                                  =                                                      ∑                    i                                    ⁢                                      (                                                                  E                        S                                            -                                              E                        i                                                              )                                                              ,                              (                                  i                  =                                                            1                      ⁢                                                                                           ⁢                      …                      ⁢                                                                                           ⁢                      n                                        -                    1                                                  )                            ,                                                          (        1        )            where ES is a target value (the set energy, as it is called) and Dn−1 is the sum of the deviations of the preceding pulse energy values from the value of the set energy. A(ES−En−1) is the proportional term of the PI regulation and B Dn−1 is the integral term.
In the technical literature pertaining to control and regulating engineering, A and B are designated as amplification constants. These are empirical values and are therefore to be determined experimentally.
As is described in U.S. Pat. No. 6,005,879, the charging voltage to be adjusted for the n-th pulseUn=Un−1−[A(ES−En−1)+B Dn−1]/(dE/dU)  (2)can be calculated from equation (1).
In this connection, dE/dU is a ratio of the change in the pulse energy of the excimer laser with variation of the charging voltage U, which ratio must be determined sequentially (at least once per burst) in order to be able to calculate with a moving average.
The disadvantage of the conventional algorithm consists in that A and B in equations (1) and (2) have fixed values which must be determined empirically at the start. However, the pulse statistics change over the gas life of an excimer laser and A and B must accordingly be optimized anew to minimum σ-values by trial and error. This involves extensive on-site measurements of the equipment by service engineers.